Under the mixed carbon policy of cap-and-trade and carbon tax, this paper considers a supply chain system composed of a manufacturer and a retailer. By using the Stackelberg game method, the decentralized decision model of a supply chain system is constructed to determine the optimal emission reduction and pricing decisions of the manufacturer and the retailer. Based on these decisions, this paper analyzes the technology selection strategy of the manufacturer in the process of low-carbon transformation, discusses the relationship between the optimal emission reduction rate of low-carbon products, carbon quota and carbon tax, which provides theoretical basis for the government to formulate policies. The research shows that the technological choice of the manufacturer in the process of low-carbon transformation mainly depends on the difference between the carbon emission cost of ordinary products and low-carbon products. When the difference is less than the threshold value, the two products will be produced together, otherwise, ordinary products will be discontinued. The government can promote optimal reduction rate of the low carbon product by adding carbon quotas. The impact of carbon tax on the optimal emission reduction rate of low-carbon products is complicated. When the relative cost of emission reduction is low, the increase of carbon tax can promote the emission reduction rate of low-carbon products, while when the relative cost of emission reduction is high, the increase of carbon tax will make the emission reduction rate increase first and then decrease. Finally, an example is given to verify the above conclusion.